Download Getting Rid of Carbon Dioxide during Exercise

Clinical Science (1996) 90, 323-335 (Printed in Great Britain)
323
Editorial Review
Getting rid of carbon dioxide during exercise
Norman L. JONES and George J. F. HEIGENHAUSER
McMaster University Health Sciences Centre, Hamilton, Canada
INTRODUCTION
As long ago as 1936, Grace Eggleton in her
textbook on ‘Muscular Exercise’ [l], wrote ‘Were it
not for the peculiar properties of carbon dioxide-a
very weak acid and a gas-our bodies would be
unable to survive in their present state. Some other
mechanism, not involving the liberation of an acid,
would have to be evolved. For the body will not
tolerate any but minor changes in the acidity of
tissue fluids.’ She went on to describe the integrated
links between metabolism, circulation and respiration in removal of CO, during exercise, already
established in the early 1920s by Meakins and
Davies [2], Douglas [3], and others. Since then, the
topic of CO, removal during exercise has never
received the same attention as 0, delivery, and
generally has been considered to be less important.
Our concern has been focused on the supply of
oxygen to fuel the metabolic fire, rather than with
the mechanisms that have evolved to deal with the
‘smoke’ of that fire [4]. This is partly due to a sense
that CO, is produced in muscle as a virtually inert
product of metabolism, diffuses rapidly into blood
and is readily transported to, and excreted by, the
lungs; and partly because the homoeostatic role of
the lungs is seldom stressed to capacity.
Of course, the linkage between CO, and 0,is so
strong as to make it almost impossible to separate
the independent effects of the various mechanisms
on each; 0,is used and CO, produced in metabolism, and the same factors influence 0,supply to
and CO, removal from muscle, and their exchange
in the lungs. Almost 30 years ago, in a series of
papers in Clinical Science [S-71, we proposed an
integrated approach to the study of the systems
involved in exercise which was based on an analysis
The central and innovative
of CO, rather than 0,.
aspects of the approach were a rebreathing method
for the non-invasive measurement of central venous
pressure (Pco,)and the quantitative assessment of
metabolic CO, production, muscle storage of CO,,
transport of CO, by the circulation, and the
efficiency of lung gas exchange in its excretion.
More recent research has provided insights into the
linkages between acid-base control, metabolism and
muscle performance, and the factors that influence
CO, elimination during exercise. Thus the overall
objective of the present review is to re-examine
some of the current paradigms related to our understanding of CO, in exercise, and perhaps redress the
relative importance between it and 0,.
METABOLIC BIOCHEMISTRY AND CO,
The increased muscle metabolism during exercise,
as well as being the source of an increased CO,
production, also generates ionic and osmotic
changes that influence the intracellular bicarbonate
content and CO, pressure.
The oxidation of glycogen and non-esterified fatty
acids (NEFA) provides most of the energy by which
ATP is restored in exercise. For a given metabolic
energy production, much less CO, is produced in
the oxidation of NEFA than glycogen. This may be
appreciated from the stoichiometry of the reactions;
for glycolysis, 1mol of C O , is produced in regenerating 6mol of ATP:
C6Hl2O6+36ADP+36Pi+6O,+
+
36ATP 6C0,
+ 6H,O
For a representative NEFA (palmitate):
C 6H3202
+ 129ADP+ 129Pi+230, +
129ATP+ 16H20+ 16C02
shows that 1 mol of CO, is produced for 8 mol of
ATP, indicating a substantially more efficient energy
source, from the acid-base point of view. Thus the
factors that control the balance between fat and
glycogen oxidation also influence the amount of
CO, produced; dietary changes [8, 91 and the
Key words r i d h e , blood, arbon dioxide stores, muscle. phpicochemid approrch.
Abbreviations 1%l a t e ; NEFA, nowterified fatty r i d ; K r , phorphocrercine; PDH, pyruvate dehydrogenase; Py. pyruvate; SID, strong ion difference.
a r m s p o w Dr N. L. Jones. Room 311%. McMaster University Health Sciences Centre, 1200 Main Street West, Hamilton, Ontario LEN 32.5. Gnda.
N. L. Jones and G. J. F. Heigenhauser
324
effects of training [lo] are two examples of changes
in CO, output at a given power output and V0,.
The other major sources of energy are phosphocreatine (PCr) and anaerobic glycolysis. The creatine
kinase reaction enables ATP to be regenerated in
the absence of glycolysis, and also acts to shuttle
energy equivalents between the cytosol and the
mitochondria [ 1 I]:
PCr2
~
+ ADP3 + H +-Cro + ATP4
~
The reaction is associated with a reduction in the
concentration of PCr2-. As the pK of PCr2- is low
(4.5) it acts as a strong acid, and its breakdown
tends to reduce muscle [H']. PCr2- is readily
available to meet demands in heavy exercise muscle;
its concentration may fall to 25% of its resting value
within 10s of starting exercise [12].
Glycogenolysis is closely linked to other changes
in muscle; the activity of phosphorylase kinase is
influenced by the release of Ca2' and by adrenaline,
leading to activation of glycogen phosphorylase
[13]. Lower in the glycolytic pathway, the activity
of phosphofructokinase is influenced by similar
changes and both enzymes are inhibited by
increases in [H'] [14]. The overall flux in the
pathway determines the rate of NADH and pyruvate production. Pyruvate either enters the citric
acid cycle or is transformed to lactate, and NADH
is oxidized aerobically in the cytochrome system or
linked to lactate production from pyruvate.
The lactic dehydrogenase reaction allows NADH
to be oxidized in the face of insufficient 0, supply
or a rate limitation in the pyruvate dehydrogenase
(PDH) reaction, which controls pyruvate (Py-)
entry into the citric acid cycle [l5]:
Py-+NADH+H'-La-+NAD+
Lactic acid meets the criteria for a strong acid,
having a pK of 3.8; its accumulation in muscle tends
to increase [H']. Since the discovery that hypoxic
contracting muscle produced lactic acid [16, 171,
lactate (La ) production by muscle has been considered synonymous with lack of O,, underpinning
the concept of the 'anaerobic threshold'. While few
would argue that a muscle deprived of 0,does not
produce lactate, studies have shown that lactate
production in heavy exercise is not always accompanied by independent indicators of lack of 0, [18,
191. It is therefore more logical to consider increases
in muscle lactate production in terms of the balance
between the rate of pyruvate formation by glycolysis
and the rate at which it is able to enter the citric
acid cycle.
Entry of pyruvate into the citric acid cycle is
controlled by PDH, a complex enzyme system
whose activity is regulated by interconversion
between two forms, one active (PDHa) and the
~
other a phosphorylated inactive form (PDHp). The
overall reaction:
Py -
+ CoASH + NAD'
+
acetyl-CoA + CO,
+ NADH
is mainly regulated by end-product inhibition by
acetyl-CoA and NADH, but its activation is mainly
influenced by Ca2+ 1203.
Finally, we should note that the products of
glycolysis, being smaller molecules than the parent
glycogen, are associated with a marked movement
of water into the active muscle cells. Intramuscular
water increases by approximately 13% in heavy
exercise, accompanied by a comparable reduction in
plasma volume. The water movements change ion
concentrations in both compartments, leading to
predictable effects [21].
Control of metabolic pathways
While this topic is beyond the scope of this
review, and is comprehensively reviewed by Newsholme and Start [14], some interesting points are
worth making in relation to lactate production and
the influence of CO, and H'. There are many
common factors acting to increase flux through
glycogenolysis and the citric acid cycle, including
Ca2 and changes in high-energy phosphates. However, differences in control characteristics allow
more precise regulation linked to the maintenance
of homoeostasis, including the effects of increases in
[H '1, tending to inhibit phosphorylase kinase and
phosphofructokinase, but helping to activate PDH
[22]. The reverse effects, as in severe respiratory
alkalosis [23, 241, may also be important in increasing lactate production, for example during exercise
at extreme altitude [25]. Differential effects on ratelimiting enzymes probably also underlie the preferential use of fat rather than carbohydrate after
training and in the carbohydrate deprived state.
Thus, whereas reductions in CO, production after
training have been ascribed to a reduction in lactate
production and the associated bicarbonate buffering
[26], the recent isotopic studies of Coggan et al.
[lo] have shown that reductions in the aerobic
production of CO, are probably of greater importance. When compared with values obtained before
12 weeks of training, their subjects showed a 14%
fall in the whole body rate of CO, appearance,
consistent with a shift towards fatty acids as fuel for
exercise.
Dalziel and Londesborough [27] showed that
changes in the content of CO, may influence the
rate of reaction of enzymes in which CO, and
NAD'/NADH are involved. 'Such enzymes include
two in the citric acid cycle, isocitrate dehydrogenase
+
isocitrate3
~
+ NAD+-a-ketoglutarate2 +
NADH +CO,
Carbon dioxide in exercise
and oxoglutarate dehydrogenase
a-ketoglutarate, -
+ CoASH +NADw
succinyl-CoA - + NADH + CO,
Dalziel and Londesborough [27] followed up an
early study by Krebs and Roughton [28], who
showed that CO, production by yeast carboxylase
was slowed by inhibition of carbonic anhydrase.
They added varying concentrations of carbonic
anhydrase to reaction mixtures to show that CO,,
rather than HC03-, had marked effects on enzyme
activity. Thus an accumulation of CO, in muscle
may reduce the maximum rate of flux through the
tricarboxylic acid cycle. This work appears to have
been largely ignored by exercise physiologists, but
its implications may be extremely important. In
much the same manner as it influences the combination of 0, with Hb [29], C O , appears to exert its
effect on carboxylase activity by the formation of
carbamino groups with amino acids close to the
regulatory subunits of these enzymes [30]. Studies
in horses [31] have shown marked increases in
muscle [La-] during exercise under conditions of
carbonic anhydrase inhibition, when the hydration
of C o t is slowed and PCO, thereby increased; other
animal studies in which Pco, has been elevated
under conditions of controlled pH have shown
reductions in tissue lactate production [32]. Reductions in muscle pH are known to have inhibitory
effects on glycolysis, but differences have been noted
between the effects of metabolic and respiratory
acidosis on exercise related changes in [La-] [333,
which possibly could be mediated by CO,.
Although the biochemical reactions associated
with CO, formation are well understood, and
several studies have demonstrated metabolic effects
associated with experimental changes in PCO, and
[HC03-], the role of CO, in each of its forms in
metabolic regulation remains to be elucidated. The
classic experiments of Jacobs [34] showed that CO,
enters cells to influence pH in the unhydrated state;
although HCO, - exchange between fluid compartments is thought to occur, the physicochemical
relationships outlined above make it clear that this
cannot occur without the movement of another ion,
and it seems more likely that apparent HC03transport is always accomplished by CO, diffusion
secondary to changes in PCO, gradients, themselves
initiated by changes in the strong ion difference in
one or more compartments.
PHYSICOCHEMICAL FACTORS INFLUENCING
HOMOEOSTASIS
The factors influencing acid-base status were
established in the early years of this century, when
325
t
4
[SID]-[HCOI] -[A-]
= [OH-] -[H+]
Fig. 1. 'Gamblegram' to show the ionic variables (cations in left
histogram and anions on the right) contributing independmtly to
[HCO,Y (shaded), within the constraint of electrical neutrality
(represented by the equal height of the two columns). Equations
present the quantitative effects d the independent variables (boxed) on
dependent variables (H', HCOI- and A-).
Henderson, Hasselbalch, van Slyke and others
applied physicochemical principles to body fluids.
More recently, Stewart [35] pointed out that the
complex quantitative relationships between systems
could be easily handled by computer. In the last 10
years we have learnt much about the factors
influencing [H '1 during exercise by applying his
approach, and the reader is referred to recent
reviews for details of these studies [36-381.
Three main systems influence [H']
and
[HC03-] in body fluids: the strong ion difference
([SID'], the sum of strong cations minus the sum
of strong anions), the concentration of weak acids
or true buffers ([AJ)
which are always in the
partially dissociated state in physiological solutions,
and the PCO, [39] (Fig. 1).
In physiological fluids the strong (fully dissociated) ions may be inorganic (mainly K', Na',
Cl-), or organic and almost fully dissociated (such
as La- and PCrZ-), having pK values that are
outside the range encountered physiologically
(below 6). The concentrations of strong ions in any
fluid compartment are influenced by the compartment's water content and by the active or passive
transfer of ions between compartments. In the case
of organic strong ions a number of processes may
occur. First, they may be formed in metabolism
from larger molecules that carry no charge, as in
La- formation from glycogen. Second, strong ions
may be transformed into weaker ions, as in the
formation of Cro and P2- from P O 2 - . Third, ion
translocation may occur between compartments,
depending on their size and on the activity of
411.
specific transporter channels [a,
Physiological buffers are nearly all weak acids;
they exist in a partly dissociated state as expressed
in the reaction:
N. L. loner and G. J.F. Heigenhauser
326
(4
HAeH' +A-
Plasma
H'
OH-
Their pK (log l/KJ values are close to physiological pH, as defined by the following equation
expressing weak acid dissociation:
200
50
K,[HA] = [H'][A-]
Thus the extent of their effect on [H'] (and also
[HC03- 3) depends on their total ([HA] [A-1)
concentration ([Alol]), and on K,. There is some
controversy regarding the value of [A,,,] and K , in
both plasma [42] and muscle, but values of
2.43 x [total plasma protein in g/dl] mEq/l, and
3.0 x lo-' respectively for plasma, and 170mEq/l
respectively for resting muscle have
and 2.0 x
been shown by titration and other validation studies
[38] to be reasonable estimates. In erythrocytes, Hb
forms most of the [A,,,], and K , varies with its
(pK 6.6) when
state of oxygenation, being 2.5 x
oxygenated and 6.3 x lo-' (pK 8.2) when fully
deoxygenated [43].
The CO, system in acid-base equilibrium may be
described by the mass action, Henderson equation:
+
[H+]= K , x P c O ~ / [ H C O ~ - ]
with the carbonic anhydrase reaction being assumed
to have reached equilibrium during the time frame
being considered:
CO, +H,OdH,CO,*H'
::I
Muscle
mEall
~
Pcq = 45 mmHg
Pco, = 40 mmHg
DH= 7.40
[H '1 =40nEq/l
pH =7.0
[H'J= IWnEq/l
HCO i
! : I50
Pco, = 30 mmHg
pH = 7.25
[H '1 = 55 nEq/l
pH = 6.5
[H '1 = 300 nEq/l
+HC03
Carbonic anhydrase activity is a crucial factor in
allowing CO, hydration to proceed rapidly enough
for the needs of heavy exercise [44].
The total amount of CO, in any fluid is the sum
of [HCO,-] and the dissolved CO,:
where s is the solubility constant (0.0307
mmol I ' mmHg- I ) [45].
Together with [H'], [HC03-] is dependent on
all three independent variables ([SID'], [A,,,] and
Pco,), with the systems being assumed to be in
equilibrium. The interaction between the three
systems may be expressed mathematically and
graphically.
The relevant equations may be solved using variations on the computer program described by
Stewart [46]; alternatively, a pictorial approach
used by Gamble [47], more recently known as
'Gamblegrams', may be used to describe the factors
involved (Figs. 1 and 2). Gamblegrams show graphically that [ H C 0 3 -3 in physiological fluids is always
the difference between the independent variable
[SID'] and [A-1, itself dependent on [A,,,], K ,
and the interactions influencing [H '1. Mathematically, the effects of changes in [SID'], [A,,,] and
Fig, 2. Gamblegrams to contrast variables influencing acid-base
state in arterial plasma and muscle, at rest (I)and after maximal
exercise (b)
Pco, on the dependent variables may readily be
calculated for muscle intracellular fluid and plasma
(Figs. 3 and 4).
CO, content
CO, content (C02,01)in muscle and plasma is the
sum of dissolved CO, and [HCO,-]. Plasma CO,
content (Cc02p,)may be calculated from a rearrangement of the Henderson-Hasselbalch equation:
CC02,,=2.226 x s x PCO,( 1 + 10pH-pK')
where s is the solubility constant for CO,, 0.0307.
The content of CO, in whole blood (cco2,b)
may be calculated by modifications of the equation
of Visser [48] as a function of plasma pH, Pco,,
Hb and arterial 0, saturation, as validated by
Douglas et al. [49]:
cCO~,~=CCO2,~(1
-[(0.0289
X
Hb)/
(3.352-0.456 x SOJ(8.142 - pH)])
m
Carbon dioxide in exercise
[A1 [HCOd
'1r"
[&I=IlmEq/l
plD]=MmEq/l
PH
[&J=llmEq/l ~ l D ] = M m E q / l
Pcq=OmmHg Pcq=40mmHg 7.7][
pr+l
160
140
I20
loo
80
60
It
0 0 -'.
mEq/l 10
-
40
[H+i
t
(0
Im
pcq
I40
-
8
25 30 IS 40 4S 50
I2 I4 I6 1810 2224
PDI
[&J
. 5 # 20
6.3
0
nEq/l
(mmHd
(mEq/l)
(mEq/l)
Fig. 1 Wculated effects on dependent variables in plasm of
dungs in tha independent variablcr Rob PID'] and [b].
Note
that a change in PIC)'] is accompanied by a virtually equimolar change in
[HCOI-1, and that increases related to incrertes in Pcq are small and
equal to reductions in [A-1.
Fig. 5. CO, content in whole blood, fully rrtuntd with 0, and with
Hb, I4g/dl, as a function of R a and plasma P l D T (plasma [AJ
constant IlmEq/l). Two lines present the conventional C02 'dissociation
curve' for blood fully saturated (arterial blood) and at an O1 saturation d
25% (limb venous blood in exercise).
[H+l
480
400
320
240
160
80
0
in part are reflected in the effect of pH on carbamate formation. Also, C 0 2 and diphosphoglycerate
share the same binding site on the Hb molecule.
These ionic interactions, in addition to the effect of
low venous 0,saturation, may increase the amount
of CO, carried as carbamate in venous blood
during heavy exercise to as much as 1520% [SO].
Carbamate concentration is a function of [Hb], 0,
saturation and pH [MI, all factors that are used in
the above equation as parameters.
CO, TRANSPORT IN EXERCISE
Fig. 4. Calculated effectson dependent variables in muscle intnceC
l u l u fluid of changes in ROI,[SlDT, and [&I. Note that changes in
PD+] have a smaller relative effect on [HCOI-1, kcruse of the large
changes in [A-] consequent on the much larger [&I in this compartment.
As [A,,] does not vary in anything but severe
exercise, when haemoconcentration occurs, the relationship between whole blood [CO,,, J and PCO,
may thus be calculated in terms of plasma [SID']
at different values of SaO, (Fig.. 5). The equation is
empirical, and in addition to dissolved CO, and
bicarbonate, includes CO, carried as carbamate, in
combination with Hb. Carbamate formation is
C02 + R-NHzeR-NHCOO-
+H t
where R represents the terminal amino acids on the
Hb molecule. About 5% of total blood CO, is
carried in this form at rest, combined with the
deoxygenated Hb. As CI- also binds to the same
amino acid chains in deoxy-Hb there are competitive effects between CI- and CO, binding [30] that
The responses that accompany the increases in
metabolic CO, production ( ko,) by exercising
muscle conceptually may be considered as a series
of conductances (C),influencing the flow of CO,
down its pressure gradient from muscle (Pmco,)to
inspired air (Pico,):
ko, = G x (Pmco, - Pico,)
The conductances are a product of flow (mainly
muscle blood flow and ventilation) and the relationship of PCO, to the CO, content (CCO,) of the
system in question. These relationships are
expressed in classical Fick Principle equations:
VCO, = 6x ~(PVCO,- Paco,)
being a function relating changes in blood Pco, to
changes in CCO,, and
VCO,
= VAx 1.1qPacoZ- Pico,)
where 1.16 is a constant that allows conversion of
N. L. Jones and G. J. F. Heigenhaurer
328
I
5
~
I
10
.
I
I5
~
20
I
25
~
I
30
.
I
35
CO1 content (mmol/l)
Fig. 6. Calculated muscle Ro,, as a function of CO1 content and
intramuscular [SID+], showing (circled) usual resting conditions
gas concentration to pressure and standardizes
volumes to equivalent conditions. Pico, is usually
ignored, being very close to zero in inspired air. In
this scheme, a diffusive conductance for CO, is also
ignored.
Thus, movement of CO, from the muscle to
expired air depends not only on the flow increases,
but also on the relationship between the content of
CO, and the pressure differences in Pc0, between
various body fluid compartments.
CO, in muscle intracellular fluid
From measurements of [SID'] in muscle biopsy
samples, titration studies and the assumption of a
PCO, that is close to the venous blood draining
muscle, the ionic status of muscle intracellular fluid
may be estimated, and also expressed in the form of
a Gamble diagram (Fig. 2). This shows that
[HCO,-] at rest is 12.5mmol/l; at a PCO, of
SOmmHg, the [C02,,,3 is 1.5 (0.03 x 50) mmol/l
higher, a total content of 14mmol/l. Using this as a
starting point, increases in [C02,,,] with exercise
are the result of the CO, produced by metabolism
and the CO, diffusing from muscle into venous
blood. PCO, increases as a function of [C02,01] for
given values of [SID'] and [A,,,] (Fig. 6). Reductions in [SID'] resulting from [La-] increases or
[K'] decreases in muscle intracellular fluid [Sl]
will be associated with reductions in [HCO,-] and
increases in Pco,. Increases in [SID'] due to
reductions in [PCr2-] will be associated with lesser
increases in PCO, and greater increases in
[HCO,-], and in this situation CO, will be 'stored'
in muscle.
From these considerations it may be appreciated
that changes in muscle PcO, and CO, content
during exercise mainly depend on the interaction
between the rate of CO, evolution by metabolism,
muscle blood flow removing CO, and changes in
.
the three main variables that influence [SID'] in
muscle: [PCr'-], [La-] and [K']. Interestingly,
some of these variables change linearly with power,
such as CO, production, but others do not, and at
a given power most change with time. Sahlin et al.
[52] showed that large reductions in [PCrz-] occur
at relatively low power, and early in exercise, when
little increase in [La-] occurs; for example, their
data demonstrate that in moderate exercise, a fall of
26mEq/l in [PCr'-] is usually accompanied by an
increase of 14mEq/l in [La-], which results in a net
implying that
increase of 12mEq/l in [SID'],
[HCO,-] could increase by this amount, without
tend
any increase in Pco,. Reductions in [PCr"]
to be maintained during constant or increasing
work-rate exercise, but are rapidly repleted once
exercise stops. In contrast, intramuscular [La-]
increases more or less exponentially with increasing
power, and at high power output will exceed the
equivalent potential increase in [SID'] due to
reductions in [PCr2 -3. When constant exercise is
maintained, [La-] tends to fall, probably mainly
due to increasing activation of PDH, reduction in
PDH inhibition by end-product inhibition as COz
is washed out, reduction in glycolysis and increasing
use of fat as fuel. Decreases in [K'] are mainly seen
in very heavy exercise, when they are accompanied
by other ionic changes including increases in intramuscular water [Sl].
While these interrelationships are complex and
markedly influenced by the intensity and duration
of exercise, they may be illustrated by considering
what happens during a progressive incremental
work-rate study to symptom limited maximum
power and peak VO,. The data employed to construct Fig. 7 were obtained from studies published
by Karlsson [53], Green et al. [54] and Hultman
and Sahlin [ S S ] , and show the potential for an
increase in muscle [SID'] up to a power output of
about 70% of maximum, but above this point there
is a fall. Not all the increase in [SID'] translates
into increases in [HC03-] due to the high muscle
[A,,,]; as [SID'] increases and [H'] tends to fall,
[A-] increases to exert a buffering effect. The
maximum increase in [SID'] of about 14mmol/l is
accompanied by a 6 5 mmol/l increase in [HCO,-]
and a 9-l0mmol/l increase in [A-I. Above about
70% VoZmax,
the falling [SID'] indicates that progressively more CO, has to be removed in order to
avoid potentially huge increases in muscle PCO,.
Usually, exercise of this intensity is accompanied by
hyperventilation and decreases in arterial PCO,; this
can have little direct effect on intramuscular pH
because the decreases are usually less than
IOmmHg, but the associated fall in arterial CO,
content helps to widen the venoarterial CO, difference. When exercise is stopped, the rapid regeneration of PCr2- and delayed washout of La- will
lead to a fall in [SID'] to well below resting values,
accounting for a surge in CO, output shortly after
stopping exercise. The data also demonstrate the
1
329
Carbon dioxide in exercise
0
40
60
%
80
100
R
k,,
Fig. 7. Usual changes seen in progressively increasing exercise to
peak (IaaX) 01,and in recovery (R) after Smin, for intramuscular
[Kr'q and [lay and arterial pluma [laq. Calculated changes in
intramuscular [SID'] (dashed line) and [HCO,-] (dotted) are shown. Data
adapted from [53-55].
relative changes in muscle compared with plasma
[La-], which presumably reflect the activity of the
transmembrane lactate transporter, which is itself
influenced by the ionic state in muscle and interstitial fluid; at a time when muscle [La-] has
increased to 12mmol/l, arterial plasma [La-] is
only 2mmol/l 1541. These considerations need to be
borne in mind when relationships between changes
in muscle [La-] and lC0, excretion are being
examined. At workloads of less than 75% capacity,
large falls in [PCr2-] and small increases in [La-]
lead to increases in [CO, 101] with relatively small
increases in muscle Pco,; however, increases in
[La-] at higher loads tend to increase [H'], and
thus Pco,, leading to CO, diffusion from the
intracellular fluid. Thus in very heavy exercise, PCO,
in femoral venous blood rises to well above
100mmHg [Sl].
Venous blood CO,
At the onset of exercise the increase in CO,
production increases the intramuscular content of
C02, and the intramuscular PCO, and [HC03-]
rise to extents dependent on other ionic changes
which may influence the independent variables
[SID'] and [A,,J in muscle. The increase in PCO,
will lead to diffusion of CO, into venous blood but
the associated rise in venous CO, content may be
insufficient to carry all the CO, produced. The
increase in venous CO, content for a given increase
in PCO, is mainly dependent on an increase in
venous [SID']; this is achieved through reductions
in plasma [Cl-1, but hindered by any increases in
[La-] 1511. Although conventionally the position of
the C 0 2 dissociation curve is shifted upwards by a
reduction in O2 saturation (Fig. 5), this effect is
mainly accounted for by the alkalinizing effect of
increases in plasma [SID '1, secondary to the movement of C1- from plasma into erythrocytes. Norne
et al. [56] have shown that C1- binds differentially
to oxy- and deoxy-Hb, accounting for some of the
acid-base changes associated with oxygenation and
deoxygenation. The effect of a reduction in 0,
saturation per se is small; C 0 2 content is only
0.2-0.7ml/dl higher in blood that is 25% saturated
than at 100% saturation, for a given plasma [SID']
(Fig. 5). The increase in CCO, as a function of PCO,
in the conventional in uitro dissociation curve is also
partly related to the increase in plasma [SID'], due
to reductions in [Cl-1, that occur in uitro when
blood is exposed to a high PCOz, and which may
not apply to blood in uiuo [57]. For these reasons
the actual increase in venous CCO, is less than
might be expected from the increase in Pco,, and
the venoarterial CO, content difference may be
insufficient for complete CO, removal from exercising muscle, even when the blood flow and arteriovenous 0,difference are adequate for 0, delivery.
This situation leads to a progressive increase in
muscle and venous Pco,, until the product of flow
and arteriovenous content difference equals the rate
of CO, production by muscle.
Even quite subtle changes in muscle metabolism
may have a relatively large effect on mechanisms
acting to remove CO, from muscle. For example, a
comparison of the effects of a fat versus a carbohydrate diet at an identical power output [9], showed
little effect on 0,delivery but a higher PCO, in
venous blood (75 compared with 60mmHg) with
the carbohydrate diet, due to the combination of a
higher aerobic CO production and higher venous
[La-].
,
Ventilation and arterial CO,
On exposure to a high PO, in the alveolar
capillaries, Hb is rapidly saturated. The pK of
saturated Hb is much lower than in less saturated
blood, so there is an abrupt increase in erythrocyte
[A-1, with a fall in pH and increase in Pco,. There
is rapid diffusion of CO, into plasma and alveoli,
and transport of CI- into plasma (the Hamburger
shift), leading to an increase in plasma [Cl-] and
concomitant reduction in [HC03-] and rise in
Pco,. A high venous CO, content dominates these
relationships, with the PCO, also being dependent
on these changes associated with oxygenation. However, the rapidity of changes is greater in erythrocytes than in plasma due to the action of carbonic
anhydrase, and Klocke [58] has emphasized the
dominant effect on CO, equilibration of the relatively slow exchange of CI- across the erythrocyte
membrane by the band 3 carrier protein. Hill et al.
[59] calculated that the half-time of changes in the
CO, system implied that the time for completion of
these reactions can exceed 1 s in exercise, when the
pulmonary capillary transit time may be as short as
0.4s. They concluded that pH, PCO, and HC03-
330
N. L. Jones and G. 1. F.
never have time to reach equilibrium even in quite
low level exercise [60].
The situation is made more complex by the
fluctuations that occur in blood and alveolar gas
related mainly to the breathing cycle, but also to
fluctuations in blood flow related to the cardiac
cycle. Often, complete equilibration is assumed
between PCO, in capillary blood and the alveoli,
but it is likely that there is a disequilibrium during
the breathing cycle and possibly even at the end of
the capillary in very heavy exercise. The importance
of the changes during the breathing cycle have been
self-evident for at least 50 years, but tackled by few
authors due to the complexity of the relationships.
Exceptions have been Nye [61] and Hlastala [62],
who both showed that fluctuations in alveolar and
pulmonary capillary PCO, increased to as much as
10mmHg even in exercise of modest intensity (VO,
of 2I/min). During exercise of higher intensity, the
fluctuation will be much higher, due to both
increases in venous PCO, and larger increases in
tidal volume. At a VCO, of 4l/min the fluctuations
in PCO, may be as much as 20mmHg [63]. Because
the fluctuations in blood flow are so much less than
in airflow, complete equilibration between mean
alveolar and arterial Pco, should not be expected
during heavy exercise. The extreme effects of slowed
equilibration are seen with carbonic anhydrase inhibition, when differences between alveolar and arterial PCO, of 20mmHg are seen during exercise. In an
important and ambitious mathematical treatment of
the factors influencing the kinetics of 0, and CO,
exchange at rest and in exercise, Hill et al. [60]
identified the importance of three factors limiting
the complete equilibration of CO, in blood during
its passage through the lungs: the short capillary
transit time, the absence of carbonic anhydrase in
plasma and the relatively long half-time for the
chloride shift between plasma and erythrocytes. The
difficulties involved in experimental study of these
factors clearly accounts for the paucity of subsequent work on this topic; Murphy et al. [64] used a
CO, electrode with a 95% response time of 0.8s to
record fluctuations in arterial blood flowing through
the arteriovenous shunts of anaemic patients with
renal failure during very light exercise. Although pH
fluctuations were recorded at rest and in recovery,
they were abolished during exercise. Studies in
heavier exercise will be needed before the question
of incomplete equilibration of CO, is settled. However, studies of carbonic anhydrase inhibition have
shown the possible effects that delayed equilibration
and the associated impairment in CO, removal can
have on muscle metabolism [65-671.
DISCUSSlON
The ideas expressed in this review present a
biased viewpoint of the importance of CO, removal
in the integrated physiological responses to exercise;
they are not new, but in some respects they do
Heigenhauser
represent a 'paradigm shift' in interpreting many
long-held notions. The approach may be criticized
for using a physicochemical approach to acid-base
physiology in which neither the parameters nor the
variables are known with sufficient precision. However, the approach is rigorous and in our view the
best we have, because it attempts to separate dependent from independent variables. Furthermore, its
validity has been established in various ways, of
which the simplest is the close concordance between
pH calculated from the measured independent variables and the measured pH in plasma [68] or
muscle biopsy samples [69].
These criticisms aside, there is ample evidence
that CO, removal is a complex process and that
impairment of any of the mechanisms involved may
have important implications for muscle metabolism
and the development of fatigue. Recently, the
importance of [H'] control in exercising muscle,
and the associated ionization state of key amino
acids in regulatory proteins, has been emphasized
(see [70] for a review).
Body CO, storage capacity
The capacity to 'store' CO, is a helpful adaptation in exercise [71]. CO, accumulates in muscle
and venous blood at the onset of exercise, particularly at low power output. At high exercise levels
the capacity, expressed in terms of ml CO, stored
per kg of body weight per mmHg increase in mixed
venous Pco,, becomes flatter due to the effects of
falls in pH in both muscle and plasma [72]. A
storage capacity in low-intensity exercise of
1 mlmmHg-' kg-' total body weight agrees well
with the theoretical values of muscle C 0 2 storage,
shown in Fig. 6, of 4.0mlmmHg- kg-' of active
muscle (lokg, 15% body weight) at a resting
[SID'] of llOmEq/l. Figure 6 also demonstrates
the dependence of CO, storage in muscle on intracellular [SID']. Increases in [SID'], due mainly to
decreases in [PCr'-], increase CO, storage, and
falls in [SID'], due mainly to lactate increases
and/or decreases in [K'], reduce storage. Thus at
high levels of exercise, when increases in [La-] and
decreases in [K'] occur [Sl], it is virtually impossible for muscle C0,content to increase, and there is
always a marked increase in muscle Pco2. This
helps to increase CO, diffusion from muscle, but the
actual washout of C O , depends also on blood flow
and the characteristics of the whole blood CO,
dissociation curve in the venous blood exiting
muscle.
We may conclude, in agreement with the estimates of Cherniack and Longobardo [71], that the
body's capacity to 'store' CO, during exercise is a
function of the mass of perfused active muscle and
its ionic composition, and the volume and composition of extracellular fluid; that it is maximal at low
exercise power outputs and becomes progressively
less at higher levels due to reductions in muscle and
33 I
Carbon dioxide in exercise
extracellular [SID']. Thus, recent work has helped
us to understand the ionic factors that influence
CO, storage in exercise, and identifies situations
leading to increases or decreases in storage
'capacity '.
Transient responses of h2
and k o 2
The transient responses of b, and k o , to step
function increases in power have been well established by Wasserman and colleagues [73] in studies
extending over several years. At low and moderate
exercise intensities, associated with little increase in
blood [La-], the time constant (T) for 0, was
shown to be about half that for CO, (30s compared
with a s ) , but there is a gradual increase in t for 0,
with increasing power, such that at about 60% of
maximum, T is equal for the two gases at approximately 6 0 s [74]. Above the power at which blood
[La-] increases, there is a drift in vo, with time,
paralleled by increases in VCO,. This is accompanied
by increases in muscle blood flow and a fall in
venous O2 saturation [75], and near-IR spectroscopy studies suggest there is also a fall in myoglobin saturation [76]. These studies have been interpreted [76] as indicating a limitation of 0,delivery
at high power output, associated with emux of
lactate, and an associated venous acidosis, which
aids in the unloading of 0,from Hb [77]. An
alternative explanation is that lactate is produced
when glycolytic flux exceeds the activity of PDH
early in exercise; later the associated fall in muscle
pH leads to an activation of PDH that allows a
further increase in aerobic metabolism. It is also
possible that end-product inhibition of PDH by
CO, plays a role early in exercise, which lessens
with increased C O , removal by blood flow.
Tramient changes in ventilation
Many studies have shown that in all situations in
which b, is changing relative to ko, in exercise,
'tracks' ko, more closely than VO,. These
findings have obvious implications for the ventilatory response in exercise, because any delay in CO,
reaching the lungs is likely to result in a delayed but
increased ventilatory response associated with a
high PCO, in venous blood. Casaburi et al. [74]
have shown that the time constant (T) for changes in
ventilation at the onset of exercise is about 75s at
low work rates but may be prolonged considerably
at high levels, to as much as 125s. This is a
mathematical expression of delayed hyperventilation
during sustained high-level exercise, associated with
progressive falls in arterial Pco,. While this effect
may help in the transport of CO, from muscle and
in limiting rises in muscle Pco,, it must be
mediated through factors controlling ventilation.
The control of breathing during exercise remains
controversial [78]; in addition to the effects of
Pco,, pH and Po, on chemoreceptors, the effects of
vE
increases in plasma [K'] have been invoked as
signals that modulate ventilatory control. Jennings
[79], on the basis of a series of studies examining
ventilatory responses to osmotic and temperature
changes, has argued recently that homoeostatic
control of ventilation may be viewed in the context
of the maintenance of protein ionization state. Thus,
changes in the body's ionic and osmotic state may
take part in the integrated ventilatory responses, in
addition to the factors that influence PCO, and
[SID'] in plasma and cerebrospinal fluid.
Ventilatory anaerobic threshold
The ventilatory response to increases in CO,
output associated with increases in arterial plasma
[La-] during exercise has been a useful concept in
exercise physiology and clinical physiology [80].
Although conceptually attractive, the identification
of a threshold has been disappointing practically
[81], at least in part because the increase in [La-]
with increasing exercise is a smooth curve. Work
suggesting that oxygen supply is not limiting during
exercise [18] has also weakened the concept. Furthermore, the notion of bicarbonate buffering of lactic
acid is clearly simplistic, in that the many ionic
changes intracellularly and in various extracellular
fluid compartments are involved. Increases in muscle [La-] increase PCO, and tend to aid diffusion of
CO, into venous plasma, but increases in plasma
[La-] will tend to reduce [SID'] and thus reduce
venous CO, content; high plasma [La-] also leads
to movement of La- into erythrocytes and thereby
may reduce the effectiveness of the chloride shift in
increasing plasma [HCO,-] in venous blood, and
in the opposite shift in the lungs.
The role of lactate uptake by erythrocytes [82],
muscle [83] and inactive tissues has been examined
during exercise [21, 84, 851. Theoretically the
uptake and metabolism of La- is accompanied by
regeneration of bicarbonate and a reduction in CO,
production related to 0,consumption, as expressed
in
La-+30,+HC03-+2C0,
However, the movement of La- and C1- into
inactive muscle lowers its [SID'] and increases
intracellular Pco,, leading to rapid diffusion of C 0 2
into its venous blood: the PCO, in venous blood
draining the inactive forearm may rise to 70mmHg
[85], accompanied by a large increase in [HCO,-],
contrasting with the changes across the active muscle (Fig. 8). The overall effects when assessed by
changes in arterial blood are a reduction in [La-]
accompanied by increases in both [HC03-] and
ko,.
The complexity of the ionic changes in blood and
tissues may account for differences in the identification of the threshold in different exercise protocols
N. L. Jones and G. J. F. Heigenhauser
332
10
-
86-
4-
-. d
2-
4 1
Na’
K’
*
/
12’
n
r
Rest
0
1
I
2
3
4
5
6
Time port-exercise (min)
7
8
9
1
n
i
Rest0
I
I
2
3
4
5 6
Time port-exercise (min)
1
1
-
7
8
9
3
Fig. 8. Arteriovenous differences in plasma ion concentrations after brief (30s) maximal exercise, across the active leg muscles ( I ) and the inactive
forearm (6).Lactate and bicarbonate concentrations both increase across the leg, but uptake of lactate across the inactive forearm is accompanied by a large release of
bicarbonate. Adapted from [SI] and [8S].
[86]; a sharper definition of the threshold is
obtained when the increments in workload are of
short duration, and accompanied by an increase in
h, of at least 150ml/min. Such protocols lead to
initial reductions in muscle [PCr2-] with some
storage of CO, in muscle, followed by large
increases and delayed washout of muscle [La-],
tending to increase CO, evolution and excretion at
the higher workloads [74].
What is the impact of concepts reviewed here on
our understanding of the increasing CO, output of
heavy exercise and its relationship to lactate production? Mainly that the notion of simple ‘buffering’
of lactic acid by bicarbonate, leading to the excess
CO, output, is weakened. With increasing power,
increasing aerobic muscle CO, production is partly
related to greater glycolytic compared with lipolytic
flux. At low power, glycolytic flux does not exceed
the flux through the PDH reaction, muscle [La-]
does not increase, but falls in [PCr2-] allow muscle
[HCO,-] to increase and muscle CO, content
increases. A t higher power the flux through PDH
becomes limiting, increases in [La-] reduce muscle
[SID’], and PCO, increases leading to a progressively increasing washout of the ‘stored’ CO,. Thus,
although increases in plasma [La-] accompany
increasing CO, output, the number of factors that
underlie any relationship between the two limits its
quantitative usefulness. Furthermore, although lactate production indicates a limitation in pyruvate
entry into the citric acid cycle, the inference of a
limitation to oxygen delivery to the working muscle
has to be questionable at best.
Lactate ‘paradox’
For many years physiologists have argued over
the factors influencing exercise capacity at high
altitudes or during the breathing of hypoxic gas
mixtures. Under severe hypoxia, exercise capacity is
reduced, and plasma [La-] is higher at a given
power; however, peak [La-] is lower than during
exercise at sea level, especially in the acclimatized
individual. The recent impressive series of studies
known as ‘Operation Everest II’, designed to simulate the temporal aspects of the ascent of Everest in
a hypobaric chamber, has generated considerable
data on this topic. Sutton et al. [25] studied
subjects on five occasions as inspired Po, was
Carbon dioxide in exercise
gradually reduced from ‘sea level’ (150 mmHg) to
the ‘summit’ (43mmHg). Maximal 0, uptake decreased progressively from 4.0 to 1.2 I/min, and maximal arterial [La-] decreased from 7.8 to 3.4mmol/l.
At sea level, plasma [La-] at a comparable power
to maximal effort at the summit (120W) was
1.5 mmol/l. Associated with these changes were the
effects of chronic hyperventilation; arterial PCO,
was 11.2 mmHg at rest and 9.6 mmHg at maximum
exercise, compared with 34 and 35 mmHg respectively at sea level; [HCO,-] was 9.9mmol/l at rest
and 7.8 mmol/l at maximum exercise, compared
with 22.2 and 16.5mmol/l at sea level. This degree
of hypocapnia at altitude, if translated into the
muscle intracellular space, is expected to lower
[H’] sufficiently to inhibit PDH activation [87].
Furthermore, the changes in [HCO,-] imply a
large increase in plasma [Cl -1, ivith intracellular
ionic changes that may only be guessed. Thus it
seems likely that the lactate paradox will eventually
be explained in terms of ionic changes influencing
the activity of flux-generating and rate-limiting
enzymes.
Exercise limitation in cardiac failure
The reduction in exercise capacity in patients with
heart failure has been considered primarily due to a
failure of adequate 0, delivery, as reflected in
increased blood lactate. However, recent studies
have suggested that the PO, in muscle is similar in
healthy control subjects and patients with heart
failure [88, 891, and that there is poor activation of
oxidative enzymes, associated with reductions in
muscle pH and lower levels of PCr2- [90]. Also, in
patients with impaired cardiac function, CO, washout is impaired and associated with a high venous
Pco,, increases in venous plasma [La-] and
delayed increases in ventilation [91]; the falls in
Paco, accompanying the delayed hyperventilation
help to limit increases in muscle Pco,. Uptake of
lactate by inactive muscle [85] also helps to remedy
the situation [90], but its impact on the total CO,
transport depends on the perfusion of inactive muscle which is probably a very small proportion of the
total cardiac output in such patients. Thus, there is
evidence that impaired muscle blood flow reduces
the removal of CO, from active muscle in patients
with cardiac failure, possibly contributing to reductions in metabolic flux and to increases in muscle
[H’]. This recent information has led to a renewed
interest in the factors that impair CO, removal as
opposed to 0, daivery in circulatory disorders
~921.
CONCLUSION
New concepts of metabolic regulation and the
control of ionic and acid-base aspects of the internal environment have allowed us to re-examine the
conventional concepts related to exercise limitation
333
in health and disease. In doing so, we are forced to
revise and redefine the role of mechanisms involved
in CO, removal. Whether we conclude that in many
situations it may be more critical to remove CO,
than to supply 0,is less important than keeping an
open mind on their importance, and building into
our research testable hypotheses that accord at least
an equal importance to CO, production and excretion as to 0,delivery and consumption.
ACKNOWLEDGMENT
Research in our laboratory has been supported
by the Canadian Medical Research Council and the
Heart and Stroke Foundation of Ontario; G.J.F.H.
is a Career Investigator of the Foundation.
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